The book description isn't sufficiently differentiating between the level the mech is at and how tall it is for LOS purposes. The attacking mech would be at Level 3 (the target's level of 1, +2 because the attacker is in mid-air about to make an attempt to land on it). For those firing at the attacking mech, they would be dealing with an LOS level of 5 as the example says, because the DFA'ing mech's feet are "planted" in mid-air at Level 3, but it's also two levels tall like any other standing mech.
The rules text should be altered:
Its LOS level is thus two, plus the level of either the target’s hex or the attacker’s hex (whichever is higher).
Change to:
Its level is thus two, plus the level of either the target’s hex or the attacker’s hex (whichever is higher).
The example text should be altered:
the jumping ’Mech is considered to be in Hex C, with an LOS level of 3 (the target hex’s level +2).
Change to:
the jumping ’Mech is considered to be in Hex C, with a level of 3 (the target hex’s level +2).
Good timing on this catch: I just might be able to get it into the next AGOAC printing.